Polymeric materials are increasingly being used in medical devices. In particular, silicone rubbers are being used in applications that rely on flexibility. For example, silicone rubber is used in joint replacement devices, surgical implants, and surgical stents. Silicone rubbers are also used in medical equipment used external to a patient's body, such as fluid flow devices, including tubing, pumps and valves. In each of these applications, sterility of the devices and implants is desirable.
Traditional sterilization techniques include autoclaving, including heating components in the presence of water or steam under pressure. Other typical sterilization techniques include radiation techniques, such as irradiating with gamma radiation. However, some types of silicone, and in particular, polyalkylsiloxanes, tend to undergo cross-linking during sterilization procedures.
More recently, the medical devices industry has increasingly turned to irradiation procedures for sterilizing medical devices. Irradiative sterilization techniques tend to cause free radical generation in non-polar silicone polymers, such as polyalkylsiloxane. Such free radical formation leads to additional cross-linking, resulting in a change in physical properties. Moreover, in devices in which two silicone surfaces are in contact, the free radical formulation and subsequent cross-linking may lead to a bonding of those surfaces.
Bonding of contacting surfaces is particularly disadvantageous in fluid flow devices, such as cannulas, valves, and duck bill shaped components. In valve configurations in which a slit or overlapping flaps act as valves, cross-linking may reduce or eliminate the opening and the ability to control fluid flow.
Traditional methods to prevent slit or opening surfaces from bonding together include application of lubricants and surface coatings on the exterior of the valves and openings. However, such an application of exterior lubricants introduces a costly and inconvenient step into the manufacturing process. In addition, the medical devices industry is turning to sterilization of prepackaged products, which limit access to the devices and prevent introduction of lubricants prior to the sterilization process. As such, improved silicone formulations and medical devices formed of those formulations, as well as, improved methods of sterilizing such medical devices would be desirable.